Acoustic transducers are generally based on piezoelectric materials. Most piezoelectric materials in common usage are also ferroelectric, i.e. they possess a permanent electric dipole, the direction of which may be reversed by the application of external stimuli such as electric fields, chemical treatment, and heat, in various combinations. Two of the most common materials for transducer construction in the acousto-optics industry, lithium niobate, formula LiNbO3, abbreviated as LN, and lithium tantalate, formula LiTaO3, abbreviated LT, are in this category. The piezoelectric element is the heart of the transducer as it converts electrical energy to acoustic energy, and vice versa. The piezoelectric is usually in the form of a thin parallel-sided plate and has thin electrodes attached to two of its opposite faces. When a periodic electric field is applied across the piezoelectric material, the local polarization in the material changes and produces a time-varying mechanical strain. It is this varying mechanical strain which propagates away from the transducer in the form of an acoustic wave.
Acoustic transducers are currently used for a variety of purposes. For example, ultrasonic transducers are used for imaging applications, such as medical imaging and industrial non-destructive testing. Acoustic transducers are also used for high power applications including medical treatment, sonochemistry and industrial processing. Ultrasonic transducers can inject ultrasound waves into the body, receive the returned wave, and convert the returned wave to an electrical signal (a voltage). Most medical ultrasound transducers are piezoelectric-based.
Devices in which an acoustic beam and an optical beam interact are generally referred to as “acousto-optic devices” or AO devices. Examples of common AO-based devices include acousto-optic tuneable filters (AOTFs), acousto-optic modulators, (also called “Bragg cells”), and acousto-optic deflectors. In most commercial AO devices, the acoustic beam is introduced into an acousto-optic (AO) interaction medium, such as TeO2, using an acoustic transducer in the form of a plate of a crystalline piezoelectric material, such as lithium niobate (LN). A top electrode is used to excite acoustic vibrations in the transducer. A metal ground electrode is used before bonding, and the metal top electrode is deposited on the upper surface, forming a structure analogous to a parallel plate capacitor. An RF generator is connected between the ground electrode and the top electrode via a suitable broadband electrical matching network to limit reflection of RF energy, and serves to produce mechanical vibrations in the plate due to the piezoelectric effect. These vibration waves pass into the AO interaction medium, where they produce changes of refractive index and hence diffraction of the incident optical fields, after which they are generally absorbed by a suitable absorber of acoustic waves to prevent acoustic reflections inside the interaction medium, which can degrade the performance of the device.
The local strength of the acoustic wave generated by a piezoelectric transducer depends on the product of (1) the local electric field strength and (2) the local piezoelectric activity, the latter being related to the crystal structure of the transducer. Usually the transducer used in acousto-optic devices is a single crystal of lithium niobate (LN). Lithium tantalate (LT) is less frequently used. A piezoelectric material such as LN is a so-called “hard ferroelectric” and it is difficult to manipulate the local piezoelectric strength in the way it is possible to manipulate the local piezoelectricity of a piezoelectric ceramic. This latter material being typically used in acoustic transducers for generation of lower frequency (tens to hundreds of KHz) acoustic waves for example in sonar applications. Thus, it is relatively easy to arrange for a sonar transducer launching an acoustic beam into water to be apodized by controlling the degree of local poling of the piezoelectric material, and so generate an acoustic beam of arbitrary spatial intensity distribution, but it is comparatively difficult to apodize the beam from a LN transducer launching an acoustic beam into an AO crystal. A designer generally has only two options in practice; to attempt alter the piezoelectric activity or to locally alter the electric field strength.
It has long been known that the domain structure of a ferroelectric material such as LN or LT may be inverted by a number of different techniques including, but not limited to, application of an electric field, heating, or chemical treatment along with heating. The inversion is generally maintained once achieved and does not require refresh cycles allowing use of this phenomena to produce ferroelectric memory. The non-volatility in ferroelectric memory is derived from two stable states of polarization and the energy barrier that must be overcome to switch between them. It is noted that all ferroelectric materials are also necessarily piezoelectric, (although the converse is not necessarily true), a consequence of the constraints of crystal symmetry at the atomic level.
A domain inverted layer can be formed on the surfaces of 36°-rotated y-cut plates of LN frequently used for longitudinal acoustic wave generation, by heating to 1100° C. for 2 hours (adjacent to positive surface, thickness of inverted layer formed approximately 160 μm). It is known to be important to ramp the temperatures up and down slowly to avoid cracking, and a temperature change of up to 50° C./minute is generally considered safe. In order to avoid loss of lithium from the surface of the LN, it is customary to flow argon gas containing water vapor over the sample during the heat cycling process.
It is generally difficult to domain-invert any LN or LT cut which contains the crystal c-axis, such as the x-cut. In comparison, inverting z-cut material is relatively easy and is routinely done at room temperature. The 36° rotated y-cut often used for longitudinal acoustic wave generation in AO has the c-axis inclined at 36-degrees to the plane of the crystal plate, and this makes it rather more difficult to invert. However, this can now be routinely inverted, such as disclosed in Nakamura K et al, “An ultrasonic transducer for second harmonic imaging using a LiNbO3 crystal”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 53, No. 3, March 2006, p 651-655 and Nakamura K, et al., “Broadband ultrasonic transducers using a LiNbO3 plate with a ferroelectric inversion layer”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 50, No. 11, November 2003, p 1558-1562.
There is significant interest in the generation and control of shear acoustic waves in AO devices, and in particular in AOTFs, and for this the z and the 36° rotated y-cuts are inappropriate because these devices require shear acoustic waves for operation, and the z and the 36° rotated y-cuts generate longitudinal waves. Usually the so-called x-cut or the 163° rotated y-cut are used to generate shear waves. The x-cut has the c-axis actually in the plane of the plate. However, it has been demonstrated that the x-cut can be inverted by depositing metal electrodes on one side of the plate and applying an electrical voltage between the electrodes while heating, in such a way that the c-axis is reversed in the region between the electrodes. The resulting inversion layer extends to a depth typically of a few tens of μm. Inversion in the other shear cut, the 163°-rotated y-cut, is only slightly more difficult than the 36° rotated y-cut as the c-axis is inclined to the plane of the plate by 17°, approximately one half of the value for the 36° rotated y-cut, but generally still enough to work with.
Although AO devices having transducers which include a domain inverted layer are known, such transducers do not significantly increase the radiation resistance, which is desirable in large area acoustic transducers because it facilitates broadband electrical matching and permits generation of spatially varying acoustic wave fields.